Claude handle undercounted credits for cancelled requests

[pwang347]

Fixes https://github.com/microsoft/vscode-internalbacklog/issues/7754
Depends on microsoft/agent-host-protocol#267

[Copilot]

Pull request overview

This PR improves per-turn Copilot credit accounting for Claude-backed chat sessions by ensuring credits that become known after a turn is cancelled/disconnected are still attributed to the correct (now-terminal) turn, rather than being dropped and undercounted.

Changes:

• Report copilot_usage.total_nano_aiu to the agent host even when a /v1/messages streaming request is abandoned mid-stream, if the proxy already observed usage before cancellation.
• Allow ChatUsage actions to amend usage on terminal turns (not just the active turn), and merge usage so “credit-only” updates don’t erase previously reported usage data.
• Add reducer tests covering cancellation + late usage attribution and idempotent “running total” updates.

Show a summary per file

File	Description
src/vs/platform/agentHost/test/node/reducers.test.ts	Adds tests for ChatUsage attribution to terminal turns and idempotent credit updates.
src/vs/platform/agentHost/node/claude/claudeProxyService.ts	Reports already-observed credits on streaming cancellation paths to avoid undercounting.
src/vs/platform/agentHost/node/claude/claudeAgentSession.ts	Tracks turn lifecycle and re-emits credit usage when credits arrive after a turn ends.
src/vs/platform/agentHost/common/state/protocol/channels-chat/reducer.ts	Applies ChatUsage to active or terminal turns and merges usage rather than replacing it.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 4